Monday, May 12, 2014

How we learn from Surprises – Part II

We owe an awful lot to the scientific method.   It is a remarkably simple concept but it is profoundly important to the technological progress of our society.   My admitted simple view of the method is below:

1. State the problem or premise
2. Form a hypothesis
3. Experiment and observe
4. Interpret the data
      If you are really surprised go back to #1
5. Draw conclusions and make predictions

This is essentially what we did when we tested our new wallboard room chamber for ventilation rate with the hypothesis that it would be quite low versus the normal infiltration rate for residences of about 0.4 mixing air changes per hour.   We got a real surprise in step 4 when it was over 10 times higher than expected and we started over at step 1. This entire story is presented in a previous blog.  In this blog I want to discuss another Step 4 surprise.

We were working on an exposure assessment of a wood preservative that was used to preserve wood that was being used indoors.  We needed to understand the rate of off gassing of this preservative so that we would estimate the indoor airborne concentrations in various scenarios. 

We knew that absorbent materials within typical rooms (carpets, furniture, etc.)  would act as reservoirs or “sinks” so we decided that we wanted to measure the “pure” rate of off gassing by putting a piece of treated wood into a glass chamber, ventilating the chamber and measuring the concentration in the exhaust air over time.    We originally modeled this system with the following conceptual picture of what was occurring.   First the preservative would off gas from the wood. It would then go to the air where some of it would be deposited on the glass surface and some would be exhausted from the glass chamber.  Eventually the glass “sink” would be filled and the entire system would be in equilibrium.   We assumed that the preservative was chemically stable and would not degrade in the time frame of this experiment.

We were wrong.  We could not get the model to work without putting in a degradation term for the material on the interior glass surface.   Once we did this the numbers worked out.    Declaring that there was significant chemical degradation on the glass was not sufficient.  We needed to prove it.  We did so with another experiment where we deposited a known amount of preservative on the glass, proved that we could get 100% of if back at time equal zero and then measured its degradation with time.   The sorted details of all this are provided in a 1995 paper which I will be happy to send to whomever asks me for it a

All this harkens back to the wise quote:  "All Models are Wrong but some are Useful."  This surprise was incredibly useful to us and lead to a much better understanding of the fate and ultimate exposure potential of this product indoors.  As it turns out, it was somewhat reactive with normal ambient oxygen in our atmosphere but, as you might imagine, it is very reactive to even the low levels of tropospheric ozone that can make its way into our indoor air especially through open windows and doors.

Treasure your surprises.

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